João Paulo Cabral Telo

Professor Auxiliar

Departamento de Engenharia Química
Instituto Superior Técnico
Universidade Técnica de Lisboa
Av. Rovisco Pais
P - 1049-001 Lisboa
Portugal
telefone: (+351) 21 841 7879
fax: (+351) 21 8417122
email: jptelo@ist.utl.pt

Formação Académica 

Interesses científicos

Organic Mixed-Valence radicals

Electron Paramagnetic Resonance (EPR) and Optical VIS/NIR spectroscopies are used to study Organic Mixed-Valence compounds (radical-anions) of the general formula M-B-M-, that have symmetrical charge-bearing units separated by a bridge. These are the simplest electron-transfer system known and their study enables to test current electron-transfer theories, like the well-known Marcus-Hush theory.

The studies have been centred mainly on Dinitro Arene radical-anions. Enormous versatility, achievable synthesis and reasonable stability (under anoxic conditions) make this class of organic radicals an interesting subject area on Organic Mixed-Valence chemistry. Some examples of dinitro radical anions studied are shown below.

The transition from Localised (the charge is instantaneously localised in one nitro group) to Delocalised (the charge is delocalised evenly over the two nitro groups) has been of special interest to us. Radical anions like pseudopara-dinitro[2.2.2]paracyclophane, 4,4'-dinitrostilbene and 4,4'-dinitrotolane (optical spectra in the picture) are so close to the Localised/Delocalised borderline that changing solvent makes them change class.

Other examples include extended quinone radial anions. They all show optical spectra typical of delocalised mixed valence compounds, but the long quaterphenoquinone (n=2) radical anion shows the wide and featureless optical band typical of a localised compound.

 

Redox Chemistry of Oxidants and Antioxidants


EPR spectroscopy, HPLC analysis and other techniques have been used to study molecules that may induce oxidative stress and the redox chemistry of antioxidant compounds that may prevent oxidative stress. Examples of the former are the elucidation of responses to oxidative stress in yeast upon exposure to the herbicide 2,4-dichlorophenoxyacetic acid [2,4-D]. Studies on the antioxidant effects of polyphenols like resveratrol (3,5,4'-trihydroxystilbene), present in red wine, e-viniferin and of other non-natural hydroxystilbenes, used as model compounds, enabled us to understand the effects of these powerful antioxidants on the prevention of oxidative stress.

We have also studied the potential antioxidant effect of several xanthines and investigated the mechanism of their reaction with reactive oxygen species. Uric acid is a well known antioxidant and methyluric acids are good antioxidants as long as they are unsubstituted at N7, since these compounds react with radicals by H-abstraction at N(7)-H. Most methylxantines present in the human diet (like caffeine, theophyline and theobromine) react with oxidant species, but again the antioxidant effect in enhanced for theophyline as compared with caffeine or theobromine, since this compound, found in tea, has a H at the N7 position, and not a methyl group.

 

Serviços Académicos

Tem lecionado as cadeiras de Laboratórios de Química Orgânica (I e II) e Química Orgânica I e II da licenciatura em Química, Engª Química e Engª Biológica.
Nos últimos anos tem regido a cadeira de Química Orgânica na licenciatra de Engª de Materiais, Engª Biomédica e Engª do Ambiente. É coordenador do laboratório de química orgânica.

 

Publicações Representativas

Telo, J.P., Shohoji, M.C.B.L., Herold, B.J., Grampp, G.. Intramolecular Electron Exchange in the 2,7-dinitronaphtalene Radical Anion: Electron Paramagnetic Resonance and Kinetic Data; J. Chem. Soc., Faraday Trans, 88 (1), 47, 1992.

S. Steenken, J.P. Telo, H.M. Novais, L.P. Candeias, One-electron reduction potentials of pyrimidine bases, nucleosides and nucleotides in aqueous solution. Consequences for DNA redox chemistry, J. Am. Chem. Soc. 114 (12), 4701 (1992)

J.P. Telo, A.J.S.C. Vieira, Mechanism of free-radical oxidation of caffeine in aqueous solution, J. Chem. Soc. Perkin Trans. 2, 1755 (1997).

J.P. Telo, M.C.B.L. Shohoji, Radical-anions of dinitrophenols in aqueous solution: intramolecular electron exchange and acid-base equilibria, J. Chem. Soc. Perkin Trans. 2, 711 (1998).

J.P. Telo, G. Grampp, M.C.B.L. Shohoji, Solvent effects in the intramolecular electron exchange in the 1,4-dinitrobenzene radical-anion, Phys. Chem. Chem. Phys. 1, 99 (1999).

A.J.S.C. Vieira, J.P. Telo, R.M.B. Dias, Generation of hydroxyl radical by photolysis of Mercaptopyridine N-oxides: Application to redox chemistry of purines, in Methods in Enzymology 300, 194 (1999).

C. Privat, J.P. Telo, V. Bernardes-Genisson, A. Vieira, J.P. Souchard, F. Nepveu, Antioxidant properties of trans-e -viniferin as compared to stilbene derivatives in aqueous and non-aqueous media, J. Agric. Food Chem. 50 (5), 1213 (2002).

H.M. Novais, J.P. Telo, S. Steenken, Structure of Radicals Derived from Hydroxypyrimidines in Aqueous Solution, J. Chem. Soc. Perkin Trans. 2, 1412 (2002).

M.H. Kleinman, J.P. Telo, A.J.C.S. Vieira, C. Bohne, J.C Netto-Ferreira, Transient Spectroscopy for the Photochemistry of Ninhydrin, Photochem. Photobiol., 77 (1), 10 (2003).

J.P. Telo, Radicals derived from Uric Acid and its Methyl derivatives in Aqueous Solution: An EPR spectroscopy and theoretical study., Org. Biomol. Chem., 1, 588 (2003).

S.F. Nelsen, A.E. Konradsson, M.N. Weaver, J.P. Telo, “Intervalence Near-IR Spectra of Delocalized Dinitroaromatic Radical-Anions”, J. Am. Chem Soc., 125 (41), 12493 (2003).

V. Farines, M.C. Monje, J.P. Telo, E. Hnawia, M. Sauvain, F. Nepveu; "Polyphenols as superoxide dismutase modulators and ligands for estrogen receptors", Anal. Chim Acta. 513 (1), 103-111(2004).

C.F. Correia, R.M. Borges dos Santos, S.G. Estácio, J.P. Telo, B.J. Costa Cabral, J.A. Martinho Simões, "Reaction of para-Hydroxyl-substituted Diphenylmethanes with tert-Butoxy Radical", ChemPhysChem, 5 (8), 1217-1221 (2004).

M.C. Teixeira, J.P. Telo, I. Sá-Correia; "The herbicide 2,4-dichlorophenoxyacetic acid induces the generation of free radicals and associated oxidative stress responses in yeast", Biochem. Biophys. Res. Comm., 324, 1101-1107 (2004).

(S.F. Nelsen, M.N. Weaver, A.E. Konradsson, J.P. Telo, T. Clark, “Electron transfer within 2,7-dinitronaphthalene Radical-Anion", J. Am. Chem. Soc., 126 (47), 15431-15438 (2004).

S.F. Nelsen, A. Konradsson, J.P. Telo, “Pseudo-para-Dinitro[2.2]paracyclophane Radical-Anion, a Mixed-Valence System Poised on the ClassII/ClassIII Borderline", J. Am. Chem. Soc., 127 (3), 920-925 (2005).

G. Grampp, A. Kelterer, S. Landgraf, M. Sacher, G. Gescheidt, D. Niethammer, J.P Telo, A.J.S.C. Vieira, “EPR and ENDOR Investigation of Various Wuster’s Radical-Cations in Solution: Experimental Results, Theoretical ab initio and DFT Calculations.”, Monatshefte fur Chemie., 136, 519-536 (2005).

S.F. Nelsen, M.N. Weaver, J.I. Zink, J.P. Telo, “Optical Spectra of Delocalised Dinitroaromatic Radical Anions Revisited", J. Am. Chem. Soc., 127 (30), 10611-10622, (2005).

S.F. Nelsen, M.N. Weaver, J.P. Telo, B.L. Lucht, S. Barlow “Koopmans-Based Analysis of the Optical Spectra of p-Phenylene-bridged Intervalence Radical Ions", J. Org. Chem., 70 (23), 9326-9333, (2005).

N.A. Macías-Ruvalcaba, J.P. Telo and D.H. Evans, "Studies of the electrochemical reduction of some dinitroaromatics", J. Electroanal. Chem., 600 (2), 294-302 (2007).

S.F. Nelsen, M.N. Weaver, J.P. Telo, "Solvent Control of Charge localization in11-bond bridged Dinitroaromatic Radical Anions" J. Am. Chem. Soc. 129 (22), 7036-7043 (2007).

S.F. Nelsen, M.N. Weaver, J.P. Telo, "Charge Localization in a 17-Bond Mixed-Valence Quinone Radical Anion ", J. Phys. Chem A. 111 (43), 10993-10997 (2007).

D. Zhang, J.P. Telo, C. Liao, S.E. Hightower, E.L. Clennan, "Experimental and Computational Studies of Nuclear Substituted 1,1'-Dimethyl-2,2'-Bipyridinium Tetrafluoroborates", J. Phys. Chem. A 111 (51), 13567-13574 (2007).